1. Field of the Invention
The present invention relates to a flux gate sensor in general and to a flux gate sensor for use in a flux gate compass in particular.
2. Description of Prior Art
A flux gate sensor is a device used for measuring the intensity of an external magnetic field in which the sensor is placed. In general, it comprises a saturable core, a drive winding for driving the core into and out of saturation and one or more sense windings. The sense windings provide, when properly orientated relative to the external magnetic field, a signal proportional to the strength of the external magnetic field.
In operation, when the core of the flux gate sensor is saturated, lines of flux due to the external magnetic field pass uniformly through the core. That is to say, the permeability or the reluctance of the core is such that the lines of flux due to the external magnetic field do not concentrate in the core. On the other hand, when the core is not saturated, the permeability of the core is such that the core presents a path of least reluctance to the lines of flux of the external magnetic field resulting in a concentration of those lines in the core which alters the magnitude of the flux concentrated in the core.
In practice, the above-described change in the concentration of the lines of flux due to the external magnetic field in the core is detected by the sense winding. When an external magnetic field is the earth's magnetic field and the flux gate sensor is provided with two or more orthogonally orientated sense windings, the outputs of the sense windings will provide signals which are dependent on the orientation of the earth's magnetic field relative to the sensor, i.e., sense windings. Of course, a sensor with only one sense winding will provide an output which is dependent on the relative orientation of the sense winding and a magnetic field in which it is placed. However, by using multiple sense windings, information indicative of this orientation can be obtained without moving the sensor. Thus, when constructed with multiple orthogonally orientated sense windings, a flux gate sensor may be used as a compass.
Flux gate sensors of various configurations have been made. In a prior known flux gate sensor, such as disclosed in U.S. Pat. No. 4,277,751, there was provided a saturable core, e.g. toroid, a drive winding for driving the core into and out of saturation and at least two sense windings. The sense windings were wound about opposite legs of the core.
In the operation of the prior known sensor, it is known that when the core is not saturated, the lines of flux from an external magnetic field will increase the flux concentration in that leg of the core, e.g. toroid, in which the lines of flux from the external field are in the same direction as the lines of flux generated in the core by the drive winding. Conversely, the flux concentration will be decreased in that leg of the core in which the direction of the lines of flux from the external magnetic field is opposite to the direction of the lines of flux generated in the core by the drive winding. Under these conditions, as the core is driven into and out of saturation, the time that each of the legs goes into and out of saturation will be different by a period proportional to the strength of the external magnetic field. Since each of the sense windings provide an output as their respective leg goes into and out of saturation, a measure of this period can be obtained from the sense windings to provide a signal proportional to the strength of the external magnetic field.
In another prior known flux gate sensor, such as disclosed in U.S. Pat. No. 3,541,432, there was provided a saturable core, a drive winding for driving the core into and out of saturation, a sense winding, an apparatus including a filter for providing an output from the sense winding comprising the second harmonic of the drive winding oscillator frequency and an apparatus for synchronously demodulating the second harmonic to provide a D.C. output having a magnitude proportional to the strength of an external magnetic field in which the sensor was located.
A principal disadvantage of the latter sensor is that in order to provide a reliable output from the sense winding, it is necessary to keep the core in saturation for a significant portion of the period of the oscillator, e.g. 20-50% of the pulse period of the oscillator, or the magnitude of the second harmonic from the filter coupled to the sense winding is always close to zero volts. Of course, keeping the core in saturation for the required period consumes an undesirable amount of power or requires the use of relatively expensive cores.
Another disadvantage of the latter prior known sensor is the requirement for a filter to provide the second harmonic from the output of the sense winding. Such filters are typically expensive and require adjustments to keep them operating at the desired frequency.